Scientists are excited by a discovery that shows it's likely three species of humans existed at the same time.

The work was published today in the journal, Nature.

Until now, many thought human evolution was a steady progression through the Homo genus from one species to the next.

Will Ockenden spoke to Darren Curnoe, an evolutionary biologist from the University of New South Wales.

DARREN CURNOE: This is an exciting discovery. What they found is three fragments of jawbones and part of a face and the fossils are dated to around two million years old. And why they're important is because this is a really interesting part of human evolution it's actually when we see the beginnings of the human genus, the genus homo.

And these are basically fossils that belong to that part of the evolutionary story. And they actually tell us that there were different species, different forms of early homo that were running around Africa at about this time.

WILL OCKENDEN: Because we're homo sapiens, so is this a long lost relative?

DARREN CURNOE: Absolutely, that's exactly right. And in fact this is the very beginnings of our genus of homo, of these two-footed apes that actually showed features that looked like living humans.

WILL OCKENDEN: Is this a new species that they've discovered or more just evidence to the fact that there may be a different species?

DARREN CURNOE: Yeah it's really more evidence that there was more than one species around at this time. We've actually known about homo rudolfensis since the 1970s but there's been a lot of controversy about whether or not it was really just a male of homo habilis, but in fact what this team has done is actually showed it's very likely that there were in fact two species co-existing in Africa at this really crucial time.

WILL OCKENDEN: And how have they made that distinction?

DARREN CURNOE: Well they've looked at evidence like the teeth for example, and teeth are very good for helping us to diagnose and to compare species. And what we see with homo rudolfensis, it's really quite an interesting species, because they show these hallmarks of early homo, things like brain size expansion, which of course is something that we see in ourselves, but at the same time they have really quite large molar teeth, and this is precisely what they've found in these new fossils. It's really very indicative of these new fossils being if you like homo rudolfensis, rather than homo habilis.

So this is what scientists call the plasticine epoch - a really fancy term but what we're really actually talking about is the ice ages. And even though these early populations were living in Africa and they weren't directly affected by ice necessarily or ice sheets, but what was going on at this time is that the global climate was really fluctuating quite wildly, you know on long time scales of thousands, tens of thousands of years.

But what it actually tells us when we think about these fossils and we think about the whole environment and the climate change that was going on was that an awful lot of our evolution was shaped by climate change. That these really big processes that we see during the ice age, were actually shaping these early human populations and an important part of our evolution of a species is actually keyed in to these dramatic changes that occurred during the ice age.

TIM PALMER: Darren Curnoe from the University of New South Wales was speaking to Will Ockenden.